This invention concerns how to suppress the fluttering of a web of paper or like continuous sheet of any comparable material traveling along a predefined path in a predetermined direction. More specifically, the invention relates to a method of, and apparatus for, web flutter control that take advantage of the Coanda effect, the familiar fluid dynamic phenomenon known also as wall attachment.
The present invention is believed to be of immense utility when applied to web flutter control during the splicing of webs in a web-fed rotary printing press because the web being printed is especially susceptible to fluttering while being spliced to a new web roll according to the customary practice in the printing industry. It is not desired, however, that the invention be limited to this particular application as it is no doubt adaptable for a variety of other applications that may involve machines dealing with traveling webs or like continuous sheets of any relatively pliant material in general.
Several practicable suggestions have been made in the art of controlling the fluttering of traveling webs. According to one such known suggestion, Japanese Patent No. 2,552,595, a pair of confronting, parallel walls are provided on opposite sides of a web path. Optionally, ports are formed in the walls for introducing air jets into the space therebetween, the air jets being directed perpendicular to the traveling web, although these jets are said to be ancillary in nature.
This first known method of web flutter containment relies on the airstreams created by the traveling web itself. As ambient air is entrained and drawn into the spaces between the web and the opposed wall surfaces, the air pressures will build up and balance each other on both sides of the web, keeping the same from fluttering which might otherwise occur even with the slightest fluctuations in web tension. The fluid pressures on both sides of the web will further increase with the introduction of jets from the wall ports, even more positively damping web oscillation. Thus, with or without use of the ancillary air jets, the web is purely fluid-dynamically prevented from fluttering, without contacting any stationary or mobile parts.
Offsetting this strength of the first prior art method are the difficulties arising from the need for provision of walls on both sides of the web path. The walls require means for supporting them, and the resulting apparatus becomes all the more bulky when means are provided for introduction of air jets through both walls into the space therebetween. The necessity of the walls on both sides of the web path manifests itself as a critical drawback when the apparatus is to be utilized for web flutter suppression during web splicing. At the supply roll station of a web-fed rotary printing press, for example, the space for wall installation is available only on one side of the web during splicing, the other side being occupied by a new roll against which the web now being printed is to be pressed for splicing. This prior art apparatus is therefore unapplicable to this end.
Japanese Unexamined Utility Model Publication No. 58-83346 teaches the use of a hollow structure for conveying ultrathin sheet material therethrough. At the upstream end of this hollow structure there are provided nozzles for creating two airstreams in the upper and lower parts of its interior, the upper stream being higher in velocity than the lower. Ultrathin sheet material is pneumatically transported down the hollow structure, always floating by virtue of the pressure differential caused by the difference between the speeds of the airflows on its upper and lower sides.
This prior art pneumatic transportation system is well calculated to keep ultrathin sheet material straight as it travels through the hollow structure. No consideration is, however, made as to how to keep the material from fluttering. For this reason alone the system is unfit for flutter control of traveling webs, not to mention the fact that its mechanical construction inhibits its use for that purpose during web splicing for the same reasons as have been set forth in connection with the first described prior art.
Japanese Utility Model No. 2,503,149 is explicitly designed to damp web fluttering during web splicing. Employed to this end are baffle plates for damping fluttering of the web which travels close to the new web roll to which that old web is to be spliced. Strategically positioned along the path of the old web, and on its side opposite to that where the new web roll lies, the baffles are intended to keep the old web from fluttering caused by the airflow induced along the surface of the new web roll as the later is driven at the same peripheral speed as the traveling speed of the old web.
An objection to this third prior art device is that the traveling old web tends to be drawn to the new web roll because of the high velocity airstream created by its rotation, the baffles being positioned only on the other side of the traveling web. Not only drawn, but the old web has actually come into contact with the new web roll, sticking, in the worst case, to the adhesive edge of the new web and thereby itself tearing or breaking. If not completely stuck to the adhesive edge of the new web, the old web has often lessened its adhesiveness as a result of forced contact or rubbing, possibly resulting in splicing failure or improper splicing.
As an additional disadvantage, the web must run in sliding contact with the baffles. The web has frequently had its surface marred, making it impossible to print correctly thereon and deteriorating the quality of the printing.